1. Field of the Invention
This invention relates to a III-V nitride substrate boule and method of making and using the same, as well as wafers derived from such boules, and microelectronic devices and device precursor structures fabricated on and/or in such wafers.
2. Description of the Related Art
The current lack of high quality III-V nitride substrates for subsequent deposition of nitride epitaxial layers limits the performance and slows the needed and desired development of short wavelength optoelectronic and high power, high frequency electronic devices.
For example, the current approach of heteroepitaxial growth of nitride epitaxial layers on foreign substrates, such as sapphire, is deleterious to the ultimate material quality and device functionality for the following reasons:    (1) lattice mismatch between the device layer and substrate causes a high density of performance-degrading defects in the device;    (2) thermal coefficient of expansion mismatch between the device layer and substrate introduces strain, cracking and strain-alleviating defects in the device layer;    (3) electrically insulating substrates require lateral device geometry, which can impede current flow through the device (this problem is lessened, but not eliminated for devices grown on conductive substrates such as SiC where there still exists a voltage barrier between the device layer and substrate material);    (4) large area electrical contact to p-type device layers is more difficult due to substrate-mandated lateral device geometry;    (5) heat dissipation from the device is limited by the low thermal conductivity of thermally insulating substrates, such as sapphire;    (6) the electrical characteristics of foreign substrates are not readily modifiable for specific device applications, e.g., p-doped substrates for inverted light-emitting diodes (LED) or laser diodes (LD), or semi-insulating substrates for electronic devices;    (7) the cleavability of (Al,Ga,In)N on foreign substrates is complicated by non-corresponding cleavage planes between the epitaxial film and foreign substrate; and    (8) crystal orientations other than c-plane in epitaxial films or device layers are not readily achieved, as a result of which improved material and/or device characteristics for such alternative orientations are not realized.
Efforts to produce native nitride substrate boules by conventional bulk growth techniques have been hampered by several fundamental characteristics of the III-V nitrides. Foremost, the equilibrium vapor pressure of nitrogen over these compounds at moderate temperatures is extremely high. The III-V nitrides start to decompose at temperatures less than their melting temperatures, making conventional bulk growth techniques extremely difficult. In addition, the III-V nitrides have low solubility in acids, bases and other inorganic compounds. The combination of these material characteristics has made it difficult to make III-V nitride substrates.
Nonetheless, bulk growth of III-V nitride material has been attempted via sublimation and solution growth techniques (see for example G. A. Slack and T. F. McNelly, J. Cryst. Growth, 34, 263 (1974); J. O. Huml, G. S. Layne, U.S. Pat. No. 3,607,014; P. Chen, Final Report, Contract NASW-4981, (1995); and P. M. Dryburgh, The Ninth International Conf. on Cryst. Growth, ICCG-9 (1989)), as well as evaporation/reaction techniques (see J. Pastrnak and L. Roskovcova, Phys. Stat. Sol., 7, 331, (1964)). Slack and McNelly (G. A. Slack and T. F. McNelly, J. Cryst. Growth, 34, 263 (1974)) used the sublimation technique at temperatures of 2250° C. to produce small-sized AlN crystals (3 mm diameter by 10 mm long). Rojo, et al., Materials Research Society, December, 1999 (“Preparation and Characterization of Single Crystal Aluminum Nitride Substrates”) reports production of 1 centimeter diameter boules of aluminum nitride, and preparation of a-face and c-face single crystal AlN substrates using chemical mechanical polishing to achieve atomically smooth surfaces, for deposition of AlN and AlGaN epitaxial layers by OMVPE. Ivantsov, et al., Materials Research Society, December, 1999 (“GaN 20 mm Diameter Ingots Grown From Melt-Solution by Seeded Technique”) describes the formation of GaN ingots having a volume of 4.5 centimeters3, grown from melt solution by seeded technique at temperatures of 900-1000° C., at pressures of less than 2 atmospheres, and at a growth rate of 2 mm per hour, to provide substrates for GaN homoepitaxy. Tadatomo U.S. Pat. No. 5,770,887 discloses the formation of nitride single crystal material having an XRD FWHM of 5-250 sec and thickness of at least 80 microns on oxide buffer layers, to enable etch separation of single wafers, but the resulting wafer substrates will have a limited area due to the requirement of etching laterally through the oxide buffer layer to effect separation.
The size of bulk GaN material has similarly been limited by the thermal instability of GaN at elevated temperature and limited solubility of N in Ga melts. The high equilibrium nitrogen pressure over GaN prevents it from being grown without an extremely high pressure apparatus (see J. Karpinski, J. Jum and S. Porowski, J. Cryst. Growth, 66 (1984)). The low solubility of N in Ga, namely ˜10−5 molar at 950° C., prevents the successful solution growth of GaN (W. A. Tiller et al., Final Report, “A feasibility study on the growth of bulk GaN single crystal”, Stanford U., July (1980)). Resorting to an economically unfavorable high pressure (2×104 atm.) solution growth technique has yielded very small crystals less than 70 mm2 in area and grown at growth rates of only 20 μm/hr.
The electrical characteristics of the bulk GaN material produced by conventional techniques are also limited by a high background carrier concentration in this material. The electron concentration of unintentionally doped GaN films grown by high-pressure solution techniques is greater than 1E19 cm-3 (S. Porowski J. Cryst. Growth, 189/190 (1998) 153) and prevents controllable doping of this material for specific device applications.
The lack of large, high quality seed crystals for the (Al,Ga,In)N system has led to the development of unseeded growth technologies, as described above. The small amount of work on seeded GaN growth has most commonly been accomplished on sapphire (see, for example, D. Elwell and M. Elwell, Prog. Cryst. Growth and Charact., 17, 53 (1988)) or SiC (see C. Wetzel, D. Volm, B. K. Meyer, et al., Appl. Phys. Lett., 65, 1033 (1994); and C. M. Balkas, Z. Sitar, T. Zheleva, et al., Mat. Res. Soc. Proc., 449, 41 (1997)) due to the unavailability of nitride seeds. The same problems associated with lattice and TCE mismatch, which occur for heteroepitaxy of nitrides on foreign substrates, also occur for bulk growth on foreign seeds. Cracking of the nitride during bulk growth and upon cool down to room temperature eliminates the usefulness of foreign seeds. Growth rates as high as 300 μm/hr have been reported (C. Wetzel, D. Volm, B. K. Meyer, et al., Appl. Phys. Lett., 65, 1033 (1994)) for GaN crystals produced by the sandwich sublimation technique. However, the total GaN thickness produced was only 60 μm because non-nitride seeds were employed and resulted in considerable cracking.
Single wafers of GaN material have recently been produced by growing thick GaN films on foreign substrates, which were removed after growth by heating (M. K. Kelly, O. Ambacher, R. Dimitrov, H. Angerer, R. Handschuh, and M. Stutzmann, Mat. Res. Soc. Symp. Proc. 482 (1998) 973), chemical etching (wet/dry etching) of substrates and interlayer materials (T. Detchprohm, K. Hiramatsu, H. Amano, and I. Akasaki, Appl. Phys. Lett. 61 (1992) 2688; and Y. Melnik, A. Nikolaev, I. Nikitina, K. Vassilevski, V. Dimitriev, Mat. Res. Soc. Symp. Proc. 482 (1998) 269) or physical grinding of sacrificial substrates or intermediate layers (S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, Jpn. J. Appl. Phys. 37, L309 (1998)). The costs of such labor-intensive processes inhibit their widespread application in wafer fabrication.
Accordingly, it would be a major advance in the art to provide an improved III-V nitride substrate for microelectronic device manufacture.